A 66-year-old man was arrested in a Utah nursing home Tuesday in connection with the March 21, 1977, killing of a 16-year-old McKinley High School student whose body was found on the second floor of a building on campus.

Dawn Momohara, then a sophomore at McKinley, was sexually assaulted and strangled to death. She was found on the second floor of the English building shortly after 7:30 a.m.

Officers with the Mill Creek Investigations Unit of the Unified Police Department of Greater Salt Lake arrested Gideon Castro at 7:40 a.m. Hawaii time Tuesday on suspicion of second­-degree murder, Honolulu Police Lt. Deena Thoemmes told reporters Tuesday afternoon.

Momohara was found lying on her back, partially clothed and the victim of a sex assault. She was pronounced dead at the scene. Officers found her purse, slipper, shorts and other evidence.

An orange cloth was tightly wrapped around her neck.

The morning before she was killed, Momohara got a call from an unknown man. Later that day she told her mother that she was going to Ala Moana Center with some friends.

“That was the last time that Dawn’s mother saw or heard from her daughter,” Thoemmes said.

An autopsy determined Momohara died of asphyxia due to strangulation and that the manner of her death was homicide. Momohara had injuries to her neck where the orange cloth was used to strangle her.

The city Department of the Medical Examiner determined her injuries confirmed she was raped.

Scrapings recovered from the scene and during the autopsy were examined and determined to be seminal fluid with the presence of spermatozoa.

Numerous friends, family and acquaintances of Momohara were interviewed in 1977 by HPD detectives working the case.

One male witness told investigators that the night before Momohara was killed, between 9 and 10 p.m., he and his girlfriend drove through McKinley’s campus and saw a car and a man near the English building.

The car was described as a 1974 or 1975 Pontiac LeMans, Buick Century or Buick Regal two-door hardtop with louvered rear windows, maroon bottom and white vinyl top, according to police.

A second witness confirmed the car’s description.

“The car was parked on the grass, and a male was observed walking out from the ground-floor steps,” Thoemmes said. “The witness circled his car back around, but the male and the car had already left.”

The man was described as Asian, about 18 to 22 years old, between 5 feet 5 and 5 feet, 7 inches tall and weighing about 140 to 150 pounds. He had a light tan complexion with dark, wavy shoulder-length hair that was combed back.

He was wearing a plain, light-blue collared pullover, short-sleeve shirt and dark-colored long pants.

A composite sketch of the man was released but didn’t generate any leads.

On March 28, 1977, detectives interviewed Castro, who was later identified as a suspect.

He told police he knew Momohara since late 1976, when he met her at a school dance. Castro graduated from McKinley in 1976 and told police Momohara was friends with him and his brother, William Castro.

He said Momohara used to call him when he was still in high school and that he last saw her at the McKinley carnival in February 1977 where they talked for about 15 minutes.

He told her he was in the U.S. Army Reserve and had come home to Honolulu. William Castro was also interviewed by police and told detectives that he met Momohara at a school dance with his brother.

William Castro said he never dated Momohara, but sometimes talked to her on the phone. He said he last saw her when she walked past his house three days before she died and that they talked for about five minutes. He asked her if she needed a ride to school, and Momohara declined.

Castro told police he went to campus to see a friend the day Momohara’s body was found and said he heard a girl had died.

“Despite following up on numerous leads and interviewing multiple individuals, investigators were unable to identify a suspect at that time,” Thoemmes said. “Let’s fast-forward 42 years to March 20, 2019, when the homicide cold-case detectives submitted a request to HPD’s scientific identifications section’s forensic biology unit to process several items of evidence that was recovered from the murder scene.”

A pair of blue shorts and underwear belonging to Momohara were submitted for testing. In May 2020 a DNA criminalist obtained the profile of an “unidentified male” from the sperm fraction of the samples from Momohara’s shorts.

In September 2023, police got word that William and Gideon Castro could be “potential suspects,” said Thoemmes.

William Castro was living in Chicago, and in November 2023, HPD homicide and strategic enforcement detail detectives traveled there to “surreptitiously obtain” a DNA sample from him or one of his kids.

The detectives were able to snag a sample from one of Castro’s adult children. The sample was checked against the sperm found on Momohara’s shorts, and Castro was ruled out as a suspect.

HPD detectives found Gideon Castro living in Utah; he had a child living in another state.

The Federal Bureau of Investigation and Homeland Security Investigations helped find Castro’s son, and a DNA sample was covertly acquired. The FBI ran an initial scan before sending the sample to HPD for testing and comparison.

Testing showed that “Gideon’s son was a biological child of the unidentified male” suspect in the Momohara killing whose sperm was found on Momohara’s shorts.

HPD detectives flew to Utah and secretly acquired a DNA sample from Castro. That sample matched with the DNA found on Momo­hara’s shorts.

Thoemmes thanked the FBI, HSI Investigations, the Unified Police Department of Greater Salt Lake, and Salt Lake County Sheriff Rosie Rivera and her team for help locating, arresting and holding Castro until he is extradited to Hawaii.